1. Technical Field
This invention relates generally to bandgap reference voltage circuits and, more specifically, to a bandgap reference voltage circuit with substantial noise immunity.
2. Background Art and Technical Problems
In modern electronic circuits, there is a need for a precise reference voltage or power level. The reference voltage (or power level) maintains a baseline voltage level by which other voltages, power levels, and/or signals within the integrated circuit operate. A reference voltage must be consistent and precise so that other voltages, power levels, and/or signals can rely on its value as a standard within the integrated circuit. For example, the reference voltage should be immune to temperature variations, noise from the power supply, noise from high speed switching, and the like.
Some general examples of applications that use reference voltages include: audio codecs, digital subscriber line transceivers (for example, a High bit-rate Digital Subscriber Line (HDSL) or an Asymmetric Digital Subscriber Line (ADSL)), modems, and other communications circuits.
Typically, the reference voltage is generated based on a bandgap voltage, and is referenced to a power supply voltage, such as ground. When the reference circuit is integrated with other circuits, it becomes susceptible to noise generated by such other circuits. Prior methods of preventing the corruption of the reference voltage due to noise include: using external capacitors to isolate the reference circuit from noise, physically isolating the reference circuit from other parts of the circuit (e.g., layout techniques), and using supply isolation to isolate the power supply of the reference circuit from the power supply of other circuits.
In high speed switching, for example, the prior isolation methods have failed to adequately guard against changes in the reference voltage. By way of illustration, known reference voltage circuits could have a 20 percent change in reference voltage at a frequency of only 20 MHZ. Inherently, the 20 percent change in reference voltage is in the decreasing direction. Such changes in communications circuits, for example, result in decreased transmission power which is highly undesirable in communication devices.
In addition to poor power transmission issues, the required transmitted power is specified by various industry standards. For example, the European Telecommunications Standards Institute (ETSI) standard for HDSL recites a maximum permissible variation in transmitted power of +/xe2x88x920.5 dB, which corresponds to an acceptable variation of about +/xe2x88x925% in absolute transmitted power. Since there is a direct relationship between the transmitted power and the reference voltage, it is necessary to maintain a precise reference voltage in order to satisfy the ETSI standard of +/xe2x88x920.5 dB.
Prior methods of isolating the reference voltage have failed to adequately guard against changes in the reference voltage, and have not sufficiently met the ETSI standard for absolute power transmitted. Thus, a reference voltage circuit and method for its use is needed which overcomes the shortcomings of the prior art.
In accordance with one aspect of the present invention, an improved reference voltage circuit is provided. The reference voltage circuit is substantially immune from high speed switching noise. In addition, the reference voltage circuit is substantially immune from power supply noise. A preferred embodiment of the subject reference voltage circuit includes, diode connected transistors, an operational amplifier, and a resistive element on one input of the operational amplifier configured to prevent spurious noise from creating a non-zero mean change in current across one of the diode connected transistor. In this way, the resistive element substantially reduces voltage fluctuations due to noise from being rectified by the diode connected transistor, and hence, from affecting the output reference voltage. Thus, an improved reference voltage circuit is provided that is substantially immune to noise.